1) Field of the Invention
The present invention relates to the manufacture of copolyester fibers for textile applications, and in particular relates to an enhanced copolyester fiber, which demonstrates improved dye permanence properties and dyeing characteristics. In particular, the present invention relates to employing a small amount of pentaerythritol and a combination of at least two different dicarboxylic acids, or their anhydride or ester equivalents, copolymerized with the raw materials used to make polyethylene terephthalate (PET). The present invention has deeper dye characteristics while maintaining the physical properties compared to the addition of pentaetriitol and adipic acid alone.
2) Prior Art
Copolyester has long been recognized as a desirable material for textile applications. The basic processes for the manufacture of copolyester are relatively well known. Copolyester fibers can be blended with other fibers such as wool or cotton to produce fabrics, which have the enhanced strength and durability of copolyester, while retaining many of the desired qualities of the natural fiber with which the copolyester is blended.
Conventional PET fibers are typically dyed at temperatures of about at least 265.degree. F. so as to achieve uniform dyeing, adequate dye intensity, and acceptable dye permanence. However, the conventional dyeing processes are too harsh for certain heat sensitive natural fibers, causing excess shrinkage or a loss of physical properties of the heat sensitive fibers. When conventional PET fibers are dyed under less harsh conditions, dye uptake by the polyester fiber is lessened as is dye permanence.
However, dye permanence continues to be a problem with conventional PET because it is not as easily dyeable as most natural fibers under the same conditions. Most natural fibers are dyed at low temperatures. Low temperature dyeing of conventional PET will cause the dye to rapidly fade or bleed from the polyester. While various additives have been used in the past to provide a deeper dye uptake (dye intensity) in polyester fibers, such additives have not resulted in dye permanence for PET as required for many end use applications. As a consequence, PET in which dye permanence is an objective must be dyed under conditions of high temperature, high pressure, or both.
It is known in the art that adipic acid can be added to PET to produce a copolyester with improved dyeability. Adding increased amounts of adipic acid during production of the copolyester will increase the dyeability. However, adipic acid increases the shrinkage of the fiber and weakens the strength of the fiber. Consequently, using more than about 4 wt. % (based on the amount of terephthalic acid or its ester equivalent) is not recommended.
It is also known in the art that pentaerythritol at low levels of less than about 700 parts per million (ppm) based on the weight of the terephthalic acid (TA) or dimethyl terephthalate (DMT) can be incorporated into PET for improved dyeability. However, at levels greater than about 700 ppm, the pentaerytliritol often results in decreased strength of the fiber.
U.S. Pat. Nos. 5,135,697 and 5,272,246 to Roderiguez et al disclose the incorporation into PET of 175 to 700 ppm of pentaelythntol and 1.3 to 3.1 wt. percent adipic acid (based on the amount of TA or DMT in the PET). As set forth in Table 1 of these patents, the atmospheric dye rating of PET is defined as being 100. The combination of the addition of pentaerythritol, adipic acid, and PET resulted in a copolymer with an improved atmospheric dye rating of 112. This result is better than using adipic acid and PET, or pentaerythritol and PET. The dyeing temperature was 210.degree. F., which is substantially lower than the normal dyeing temperature of 265.degree. F. for PET. This copolyester is a great advantage over PET.
Nevertheless, there continues to be a need for improved copolyester fibers that possess enhanced, superior dyeing properties, maintain the stability of the incorporated dye, and possess suitable characteristics of tenacity, shrinkage, dyeability and dye retention.